Behavior of metal oxides in the catalytic pyrolysis of poplar wood

Researchers have investigated the effect of various transition metal oxides on the final products of catalytic pyrolysis of biomass. Their findings indicated that different metal oxides render different final products in this process.

Complications and pollution which are caused by fossil fuels have motivated scientists to look for alternative sources of energy that are renewable and eco-friendly. Biomass has all of the features of a proper source of energy. It is abundant, renewable and produces very low carbon waste.

Catalytic Pyrolysis of Biomass

Catalytic pyrolysis is the most efficient process to extract energy from biomass. Despite this, most of the bio-oils that are formed during this process could not be used as fuels considering their instability and high oxygen content. Metal oxides are very often used in the formulations of the catalysts that are used for catalytic pyrolysis of biomass. In a new study, scientists from University of Jinan and University of Tabriz have investigated the effect of various transition metal oxides on the final products of the catalytic pyrolysis process, including bio-oils.

Effect of Various Metal Oxides

The metal oxides that were studied include CoO, Cr2O3, CuO, Fe2O3, Mn2O3, NiO, TiO2, V2O5 and CeO2. Researchers investigated the effect of each of these metal oxides on the distribution and component of final products. The results demonstrated that Co, Ti, V, or Mn-based catalysts promoted the formation of heavy bio-oils. The Ce, Cr, Cu or Fe-based catalysts promoted the formation of light bio-oils. Also, all of the metal oxides, especially Ti and Ni based, were able to suppress further cracking of primary products. They also increased the tar yield and simultaneously decreased the gas yield.

The metal oxides also exert their effect on separate components of bio-oils. The metal oxides, except Fe2O3, increased the production of alcohols, furan, ketones, acetic acid and phenolics in bio-oil. Fe2O3 based catalysts suppressed the formation of ethylene glycol, furan, carboxylic acids and aldehydes such as actaldehyde or hydroxyl aldehyde. Fe2O3 based catalysts did not have significant effect on hydroxyl acetone and esters.

Since ketones are more stable than aldehydes, the formation of hydroxyl acetone was increased by oxidative catalysts. Aldehydes such as hydroxyl aldehyde are very reactive and their formation was only increased by Ti or Cu based catalysts. Also, a fair amount of coke was formed over V, Mn, Cu and Co-based catalysts. These coke species were mainly polymeric with low thermal stability.

It should also be noted that all of these oxides contain multiple valences. This feature makes them able to generate oxygen vacancies that are an important part of polymerization reactions.

Zist Fan
Biomass and Bioenergy
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